Liquid compositions, which hereinafter are often called "baths" for brevity, even if used by some other method than immersion, that are in general use for treating the surface of aluminiferous metals can be broadly classified into chromate types and nonchromate types. Chromic acid chromate conversion baths and phosphoric acid chromate conversion baths are typical embodiments of chromate type treatment baths.
Chromic acid chromate conversion baths came into practical use in about 1950 and are still widely used even at present for heat exchanger fin stock and aviation vehicle components. The chromic acid chromate conversion baths contain chromic acid and fluoride as their main components, with the fluoride functioning as a reaction accelerator. These baths coat metal surfaces with conversion coatings containing some quantity of hexavalent chromium.
Phosphoric acid chromate conversion baths originated with the invention disclosed in U.S. Pat. No. 2,438,877. These conversion baths, which contain chromic acid, phosphoric acid, and hydrofluoric acid as their main components, coat metal surfaces with conversion coatings whose main component is hydrated chromium phosphate. Because these conversion coatings do not contain hexavalent chromium, they also are in wide use at present, for such applications as underpaint coatings for beverage can body and lid stock. Nevertheless, since these chromate type surface treatment baths do themselves contain toxic hexavalent chromium even though the coatings produced by them do not, hexavalent chromium-free treatment baths are desired in view of the environmental problems from disposal of the baths, rinse waters, and the like.
Typical of the inventions in the field of the chromium-free nonchromate type surface treatment baths is the process disclosed in Japanese Patent Application Laid Open [Kokai or Unexamined] Number Sho 52-131937 [131,937/1977]. The treatment bath in that reference consists of an acidic (pH approximately 1.5 to 4.0) aqueous coating solution containing phosphate, fluoride, and zirconium or titanium or both. Treatment of the metal surface with this surface treatment bath forms thereon a protective coating whose main component is zirconium or titanium oxide. (This type of coating is often called a "conversion" coating, because it is believed that it also contains cations from the substrate in the form of oxides and/or phosphates.) An advantage of nonchromate surface treatment baths is that they are free of hexavalent chromium, and this advantage has resulted in their wide use at the present time for treating the surface of drawn-and-ironed ("DI") aluminum cans and the like. However, the nonchromate baths require longer treatment times for coating formation than chromate surface treatment baths. Shortening surface treatment times has become an important issue in the last few years, because of the increasingly high line speeds being used to boost productivity. Moreover, nonchromate baths yield coatings with a corrosion resistance and paint adherence inferior to those of chromate coatings.
The treatment process disclosed in Japanese Patent Application Laid Open [Kokai or Unexamined] Number Hei 1-246370 [246,370/1989] is an invention whose object is to shorten the aforementioned surface treatment times. In this process, the aluminiferous metal surface is first cleaned with an alkaline degreaser and the cleaned surface is then treated with an acidic (pH 1.5 to 4.0) aqueous solution containing 0.01 to 0.5 g/L of zirconium ions, 0.01 to 0.5 g/L of phosphate ions, 0.001 to 0.05 g/L, measured as its stoichiometric equivalent as fluorine atoms, of "free" fluoride ions, and optionally 0.01 to 1 g/L of vanadium ions. However, when this process is applied to DI aluminum cans, the resulting film does not always have a satisfactory resistance to blackening.
Another nonchromate treatment process is disclosed in Japanese Patent Publication Number Sho 57-39314 [39,314/1982]. Disclosed therein is a treatment process in which the aluminiferous metal surface is treated with an acidic solution containing hydrogen peroxide, one or more selections from zirconium and titanium salts, and one or more selections from phosphoric acid and condensed phosphoric acids. However, this treatment bath is unstable, and, in addition, is also inadequately rapid in terms of surface coating formation. Moreover, this document does not provide a specific description or disclosure of the treatment time, treatment temperature, or treatment process.
It is for these reasons that nonchromate type surface treatment baths are at present almost never used on surface treatment lines for aluminiferous metal coil or sheet where short treatment times are critical.
In summary, then, there has yet to become established in the art a composition or process for treating the surface of aluminiferous metals that can provide short treatment times and is capable of forming a highly corrosion-resistant and strongly paint-adherent coating, but is free of hexavalent chromium.